This invention relates to newly identified polynucleotides, polypeptides encoded by such polynucleotides, the use of such polynucleotides and polypeptides, as well as the production of such polynucleotides and polypeptides. More particularly, the polypeptides of the present invention are human homologs of the prokaryotic mutL4 gene and are hereinafter referred to as HMLH1, HMLH2 and HMLH3.
In both procaryotes and eucaryotes, DNA mismatch repair plays a prominent role in the correction of errors made during DNA replication and genetic recombination. The E. coli methyl-directed DNA mismatch repair system is the best understood DNA mismatch repair system to date. In E. coli, this repair pathway involves the products of the mutator genes mutS, mutL, mutH, and uvrD. Mutants of any one of these genes will reveal a mutator phenotype. MutS is a DNA mismatch-binding protein which initiates this repair process, uvrD is a DNA helicase and MutH is a latent endonuclease that incises at the unmethylated strands of a hemi-methylated GATC sequence. MutL protein is believed to recognize and bind to the mismatch-DNA-MutS-MutH complex to enhance the endonuclease activity of MutH protein. After the unmethylated DNA strand is cut by the MutH, single-stranded DNA-binding protein, DNA polymerase III, exonuclease I and DNA ligase are required to complete this repair process (Modrich P., Annu. Rev. Genetics, 25:229-53 (1991)).
Elements of the E. coli MutLHS system appears to be conserved during evolution in procaryotes and eucaryotes. Genetic study analysis suggests that Saccharomyces cerevisiae has a mismatch repair system similar to the bacterial MutLHS system. In S. cerevisiae, at least two MutL homologs, PMS1 and MLH1, have been reported. Mutation of either one of them leads to a mitotic mutator phenotype (Prolla et al, Mol. Cell. Biol. 14:407-415 [1994]). At least three MutS homologs have been found in S. cerevisiae, namely MSH1, MSH2, and MSH3. Disruption of the msh2 gene affects nuclear mutation rates. Mutants in S. cerevisae, msh2, pms1, and mlh1 have been found to exhibit increased rates of expansion and contraction of dinucleotide repeat sequences (Strand et al., Nature, 365:274-276 (1993)).
It has been reported by various laboratories that a number of human tumors such as lung cancer, prostate cancer, ovarian cancer, breast cancer, colon cancer and stomach cancer show instability of repeated DNA sequences (Han et al., Cancer, 53:5087-5089 [1993]; Thibodeau et al., Science 260:816-819 [1993]; Risinger et al., Cancer 53:5100-5103 [1993]). This phenomenon suggests that lack of the DNA mismatch repair is probably the cause of these tumors. Little was known about the DNA mismatch repair system in humans until recently, the human homolog of the MutS gene was cloned and found to be responsible for hereditary nonpolyposis colon cancer (HNPCC), (Fishel et al., Cell, 75:1027-1038 [1993] and Leach et al., Cell, 75:1215-1225 [1993]). HNPCC was first linked to a locus at chromosome 2p16 which causes dinucleotide instability. It was then demonstrated that a DNA mismatch repair protein (MutS) homolog was located at this locus, and that Cxe2x86x92T transitional mutations at several conserved regions were specifically observed in HNPCC patients. Hereditary nonpolyposis colorectal cancer is one of the most common hereditable diseases of man, affecting as many as one in two hundred individuals in the western world.
It has been demonstrated that hereditary colon cancer can result from mutations in several loci. Familial adenomatosis polyposis coli (APC), linked to a gene on chromosome 5, is responsible for a small minority of hereditary colon cancer. Hereditary colon cancer is also associated with Gardner""s syndrome, Turcot""s syndrome, Peutz-Jaeghers syndrome and juvenile polyposis coli. In addition, hereditary nonpolyposis colon cancer (HNPCC) may be involved in 5% of all human colon cancer. All of the different types of familial colon cancer have been shown to be transmitted by a dominant autosomal mode of inheritance.
In addition to localization of HNPCC, to the short arm of chromosome 2, a second locus has been linked to a pre-disposition to HNPCC (Lindholm, et al., Nature Genetics, 5:279-282 (1993)). A strong linkage was demonstrated between a polymorphic marker on the short arm of chromosome 3 and the disease locus. It was also suggested that these families show signs of a general defect in the DNA repair process.
This finding suggests that mutations on various DNA mismatch repair proteins probably play crucial roles in the development of human hereditary diseases and cancers.
HNPCC is characterized clinically by an apparent autosomal dominantly inherited predisposition to cancer of the colon, endometrium and other organs. (Lynch, H. T. et al., Gastroenterology, 104:1535-1549 (1993)). Tumors from HNPCC patients are characterized by widespread alterations of simple repeated sequences (microsatellites) (Aaltonen, L. A., et al., Science, 260:812-816 (1993)). This type of genetic instability was originally observed in a subset (12 to 18% of sporadic colorectal cancers (Id.). Studies in bacteria and yeast indicated that a defect in DNA mismatch repair genes can result in a similar instability of microsatellites (Levinson, G. and Gutman, G. A., Nuc. Acids Res., 15:5325-5338 (1987)), and it was hypothesized that deficiency in mismatched repair was responsible for HNPCC (Strand, M. et al., Nature, 365:274-276 (1993)). Analysis of extracts from HNPCC tumor cell lines showed mismatch repair was indeed deficient, adding definitive support to this conjecture (Parsons, R. P., et al., Cell, 75:1227-1236 (1993)). As not all HNPCC kindred can be linked to the same loci, and as at least three genes can produce a similar phenotype in yeast, it seems likely that other mismatch repair genes could play a role in some cases of HNPCC.
hMLH1 is most homologous to the yeast mutL-homolog yMLH1 while hMLH2 and hMLH3 have greater homology to the yeast mutL-homolog yPMS1 (hMLH2 and hMLH3 due to their homology to yeast PMS1 gene are sometimes referred to in the literature as hPMS1 and hPMS2). Both the hMLH2 gene on chromosome 2q32 and the hMLH3 gene, on chromosome 7p22, were found to be mutated in the germ line of HNPCC patients. This doubles the number of genes implicated in HNPCC and may help explain the relatively high incidence of this disease.
In accordance with one aspect of the present invention, there are provided novel putative mature polypeptides which are hMLH1, hMLH2 and hMLH3, as well as fragments, analogs and derivatives thereof. The polypeptides of the present invention are of human origin.
In accordance with another aspect of the present invention, there are provided polynucleotides (DNA or RNA) which encode such polypeptides.
In accordance with yet a further aspect of the present invention there is provided a process for producing such polypeptides by recombinant techniques.
In accordance with yet a further aspect of the present invention, there is provided a process for utilizing such polypeptide, or polynucleotide encoding such polypeptide, for therapeutic purposes, for example, for diagnostic and therapeutic purposes.
These and other aspects of the present invention should be apparent to those skilled in the art from the teachings herein.